There is a wide use of amplifiers in today's electronic circuits. One type of amplifier, known as a “pre-amplifier”, is typically used to amplify (“preamplify”) an unrefined, weak signal that is received from a source, such as a microphone transducer. The pre-amplifier amplifies the input analogue audio signal to provide a larger-amplitude “preamplified” output signal that has the same waveform as the input signal, within a particular tolerance. Similarly, with a received audio signal, a low level audio signal may then be applied to a pre-amplifier and the preamplified signal then passed on to a further audio power amplification stage, to drive an output device, such as a loudspeaker.
Noise may accompany the input signal if the input lines that deliver the input signal pick up noise from a source external to the integrated circuit (IC). Such noise may emanate from, for example, a noisy direct current (DC) power supply that powers a transducer, such as a microphone. Hence, most electronic circuits amplifying alternating current signals, such as audio signals, require de-coupling of the DC effects from the AC signal.
Referring now to FIG. 1, a known single-ended audio circuit decoupling a DC signal from an AC audio signal is illustrated. In this respect, FIG. 1 discloses a classic topology where there exists two differing common mode voltages in the audio signal path. Notably, the classic circuit topology requires pads and external components.
Audio Power Amplifiers are especially used to drive low impedance loads. The difference in the preamplifier and Power amplifier are especially in the current capability. The pre amplifier is able to drive a couple of 100's of uA, while the Power Amp. is able to drive a couple of 100's of mA.
An input signal 105 is input via an input resistance 110 to a first amplification stage 115 that is operating in common mode using a Vreg/2 input 120. A feedback resistor 125 completes the first stage.
The first amplification stage 115 is operably coupled to a second common mode amplification stage 155, 165, which operates in common mode using a Vbatt/2 input 160. Two integrated circuit pins 130, 135 and an external (large) capacitor 140 are required in order to avoid creating an undesired DC offset in connecting the two stages. Undesired DC offsets are created due to the output of the first stage being Vreg/2 and the second stage regulating its input of the Amp to Vbatt/2. Thus, without the use of the two integrated circuit pins 130, 135 and an external (large) capacitor 140 an undesired current of (Vbatt/2−Vreg/2)/R would be passed through the input resistor R 145 of the second amplification stage 155. This current flows also through the feedback resistor (2R) 150 of the second stage and results into a common mode voltage at the output of the second amplification stage 155. Notably, this voltage is Vbatt/2+I*2R and not at a desired mid-range value.
Due to the fact the output is not at a mid-range value, having a DC signal passed through the amplifier chain would result in clipping of the input signal as soon as it becomes higher than:Vbatt−(Vbatt/2−Vreg/2)*2R/R.  [1]
Amplifier 165 is an inverting amplifier and uses the resistors 175 and 170 to buffer the signal at the output of the amplifier 155. Thus, the loudspeaker 180 then sees the same signal as the output of amplifier 155, but in an opposite phase.
Referring now to FIG. 2, a known differential circuit decoupling a DC signal from an AC audio signal is illustrated. In this respect, FIG. 2 illustrates a classic high performance topology. Notably, the two stages have different common mode voltages. Furthermore, the classic differential circuit topology requires four pads and two external capacitors.
An input signal 205 is input to a low voltage first differential common mode amplification stage 215, 225 via an input resistor (R) 210. The low voltage differential common mode amplification stage 215, 225 operates in common mode using a Vreg/2 input 235 applied through resistor 240. Feedback resistors (2R) 220, 245 set the gain of the first stage, with the common mode inputs being provided via the feedback resistors (R) 230.
The first amplification stage 215, 225 is operably coupled to a second differential common mode audio power amplification stage 282, 284, which operates in common mode using a Vbatt/2 input 294. Four integrated circuit pins 250, 260, 265, 275 and two external (large) capacitors 255, 270 are used in order to avoid creating an undesired DC offset in connecting the two stages. Undesired DC offsets are created due to the output of the first low voltage amplification stage being Vreg/2 and the second audio power amplification stage regulating its input of the amplifier to Vbatt/2. Thus, without the use of the four integrated circuit pins 250, 260, 265, 275 and two external (large) capacitors 255, 270 an undesired current of (Vbatt/2−Vreg/2)/R would be passed through the input resistors (R) 278, 290 of the differential common mode audio power amplification stage 282, 284. The current also flows through the feedback resistors (2R) 280, 292 of the differential common mode audio power amplification stage 282, 284. Notably, the common mode voltage at the output of the differential common mode audio power amplification stage 282, 284, is Vbatt/2+I*2R, and not at a desired mid-range value.
Thus, in the audio circuits of FIG. 1 and FIG. 2, component count is increased due to the need to use one or more external capacitor(s). Furthermore, PIN count on the integrated circuit (IC) is increased due to the need to connect the external capacitor(s).
Finally, as the impedance range of the transducer is low, and as the decoupling capacitances must behave like a short circuit for a low frequency audio signal (20 Hz), the value of the decoupling capacitor must be very large (for example of the order of 100 nF). This makes the implementation both very expensive and requires an increased are on the printed circuit board (PCB).
Furthermore, the product manufacturers who utilize such audio amplifiers, such as mobile phone manufacturers, are requiring improvements in reducing noise. This is particularly the case for provision of future technologies and features where audio sensitivity to noise has been identified as a key user requirement. One solution to the aforementioned problems could be to introduce a voltage-to-current (V-I) converter with the low-voltage amplifier and a current-to-voltage (I-V) converter with the power amplifier. However, such a solution fails to meet stringent noise specifications.
Thus, there exists a need to improve the decoupling performance in audio amplifier systems, whilst not impacting noise specifications.